Compact near-IR and mid-IR cavity ring down spectroscopy device

ABSTRACT

This invention relates to a compact cavity ring down spectrometer for detection and measurement of trace species in a sample gas using a tunable solid-state continuous-wave mid-infrared PPLN OPO laser or a tunable low-power solid-state continuous wave near-infrared diode laser with an algorithm for reducing the periodic noise in the voltage decay signal which subjects the data to cluster analysis or by averaging of the interquartile range of the data.

PRIORITY INFORMATION

This application claim the benefit under 119(e) of the filing date ofthe provisional application filed 8 Nov. 2005, U.S. Ser. No. 60/734,275.

FIELD OF THE INVENTION

This invention relates to a compact continuous wave cavity ring downspectrometer for detection and measurement of trace species in a samplegas.

BACKGROUND OF THE INVENTION

The availability of compact and efficient spectroscopic quality tunablediode lasers has generated interest in the development of portableoptical diagnostic instruments. Advances in the communications industryhave produced inexpensive, reliable and robust diode lasers in the nearinfrared. In the area of trace gas detection, the use of sensitive insitu diagnostics enables improved field measurements and better processcontrol in a wide variety of applications, such as environmentalmonitoring, process control, and medical diagnostics.

Many technologies are available for measuring trace species of a samplegas, but there are tradeoffs between accuracy, sensitivity, selectivity,size and cost. Absorption spectra resulting from methods such as tunablediode laser absorption spectroscopy (TDLAS) with wavelength or frequencymodulation, and FTIR are usually easy to interpret and are not limitedby species selectivity. However, these are generally orders of magnitudeless sensitive than laboratory techniques such as GC/MS, laser inducedfluorescence (LIF), and photoacoustic spectroscopy (PA).

Cavity ringdown spectroscopy (CRDS) is a highly sensitive linearabsorption technique that is capable of monitoring a wide range ofspecies. U.S. Pat. No. 6,842,548 to Loock et al. discloses a standardmethod and apparatus for measuring one or more optical properties of atest medium, comprising providing an optical waveguide loop comprising atest medium, illuminating the optical waveguide loop with a plurality oflight pulses, and detecting roundtrips of the light pulses at one ormore locations along the loop, wherein the detected light pulses areindicative of one or more optical properties of the test medium.Preferably, ring-down time of said light pulses is determined. Theinvention provides measures of optical properties such as absorbance andrefractive index of a test medium such as a gas, a liquid, and a solidmaterial.

Although most often performed using pulsed lasers, a number of groupsare now exploring the use of cw, solid state lasers in CRDS. Lehmann etal., Meijer et al., and Romanini et al. were the first to usecontinuous-wave lasers for CRDS. In cw-CRDS a laser beam probes anoptical cavity constructed of two highly reflective mirrors (R>0.9999).Light builds up in the cavity when the wavelength matches a cavitytransmission mode. The frequency spacing between cavity transmissionmodes is the free spectral range (FSR),FSR=1/L _(rt)  (1)where L_(rt) is the round trip pathlength of the cavity in centimeters.Once the intensity reaches a preset level, the source is terminated anda ringdown event is captured. Initial attempts at cw-CRDS employedlocking the cavity length to the laser frequency to ensure the buildupof light. U.S. Pat. No. 5,528,040 to Lehman discloses an apparatus fordetection and measurement of trace species in a sample gas. A ring downcavity cell is filled with the sample gas. A continuous wave laser emitsradiation, which is directed from the continuous wave laser to the ringdown cavity cell where it resonates. A photodetector measures radiationlevels resonated by the ring down cavity cell and produces acorresponding signal. The decay rate of the ring down cavity cell iscalculated from the signal produced by the photodetector and is used todetermine the level of trace species in the sample gas.

Romanini et al. modulated the cavity length to scan several transmissionmodes of the cavity across the laser frequency. This allowed for thebuildup of light in the cavity at any frequency without thecomplications of cavity locking. U.S. Pat. No. 6,084,682 to Zarediscloses distinct locking and sampling light beams are used in a cavityring-down spectroscopy (CRDS) system to perform multiple ring-downmeasurements while the laser and ring-down cavity are continuouslylocked. The sampling and locking light beams have different frequencies,to ensure that the sampling and locking light are decoupled within thecavity. Preferably, the ring-down cavity is ring-shaped, the samplinglight is s-polarized, and the locking light is p-polarized. Transmittedsampling light is used for ring-down measurements, while reflectedlocking light is used for locking in a Pound-Drever scheme.

An acousto-optic modulator (AOM) has been used in conjunction with athreshold circuit to shut off the light source when sufficient buildupoccurred. Paldus et al. showed that an additional benefit of using anAOM is that the first order beam generated by the device is frequencyshifted, so any light that is fed back to the laser diode source willnot result in stabilization problems caused by optical feedback. Palduset al. also developed a ring configuration which allowed for locking thecavity to the laser frequency, thus increasing the precision inringdowns and improving detection limits.

U.S. Pat. No. 5,903,358 to Zare discloses a cavity ring downspectroscopy (CRDS) system uses a free-running continuous wave (c.w.)diode laser stabilized by frequency-shifted optical feedback in thepresence of strong reflections from a high-finesse Fabry-Perotresonator. The frequency-shifted feedback stabilization eliminates theneed for tightly controlling the relative positions of the laser andresonator. Non-frequency-shifted feedback is used for linewidthbroadening. An acousto-optic modulator placed between the diode laseroutput and the resonator input frequency-shifts light reflected by theresonator input, causing the laser to cycle in phase with a period equalto the inverse of the frequency-shift. The laser diode linewidth can bestabilized from several MHz for high resolution spectroscopy of speciesat low pressures, to several hundred MHz for lower resolutionspectroscopy of species at atmospheric pressures.

U.S. Pat. No. 5,815,277 to Zare discloses the use of light that iscoupled into a cavity ring down spectroscopy (CRDS) resonant cavity byusing an acousto-optic modulator. The AOM allows in-couplingefficiencies in excess of 40%, which is two to three orders of magnitudehigher than in conventional systems using a cavity mirror forin-coupling. The AOM shutoff time is shorter than the roundtrip time ofthe cavity. The higher light intensities lead to a reduction in shotnoise, and allow the use of relatively insensitive but fast-respondingdetectors such as photovoltaic detectors. Other deflection devices suchas electro-optic modulators or elements used in conventional Q-switchingmay be used instead of the AOM. The method is particularly useful in themid-infrared, far-infrared, and ultraviolet wavelength ranges, for whichmoderately reflecting input mirrors are not widely available.

Sensitivity is also an issue. U.S. Pat. No. 6,727,492 to Ye et al.discloses an ac technique for cavity ringdown spectroscopy permits1×10⁻¹⁰ absorption sensitivity with microwatt light power. Two cavitymodes are provided temporally out of phase such that when one mode isdecaying, the other mode is rising. The system and method provides aquick comparison between on-resonance and off-resonance modes andenables sensitivities that approach the shot-noise limit.

Others have tried various data manipulations to improve results. U.S.Pat. No. 6,915,240 to Rabinowitz discloses A novel system and method fordata reduction for improved exponential decay rate measurement in thepresent of excess low frequency noise. The system and method fit thetail of a record to a straight line wherein the straight line isextrapolated to the entire record and then subtracted from the initialdata points before a logarithmic transformation is taken.

Fieldable methods for detecting and measuring chemical hazards areneeded. However, instruments that operate in the field must be able towithstand mechanical vibration and shock, and produce accurate andreliable results.

SUMMARY OF THE INVENTION

In a preferred embodiment, a compact cavity ring down spectroscopyapparatus for detection and measurement of trace species in a sample gasis provided, which comprises: a housing for said apparatus; a tunablesolid-state continuous-wave mid-infrared PPLN OPO laser within saidhousing; an acousto-optic modulator in optical communication with saidlaser for steering a first order diffraction beam of said laser and forinterrupting said beam when resonance is achieved; a ring down resonantcavity within the housing for holding a sample gas, said cavity cellreceiving said first order diffraction beam of said laser and comprisingat least two high-reflectivity mirrors, wherein said mirrors define anintracavity light path and one of said mirrors is a movable tuningmirror; a piezo transducer drive attached to the tuning mirror formodulating cavity length to maintain resonance between the laserfrequency and cavity modes; and a photo-detector within said housing,for receiving said beam from the cavity and for generating a resonancesignal and a voltage decay (ring down) signal, thereby measuring aninteraction of said sample with said intracavity beam.

In another preferred embodiment, a compact cavity ring down spectroscopyapparatus for detection and measurement of trace species in a sample gasis provided, and which comprises: a housing for said apparatus; atunable low-power solid-state continuous wave near-infrared diode laserwithin said housing; an acousto-optic modulator in optical communicationwith said laser for steering a first order diffraction beam of saidlaser and for interrupting said beam when resonance is achieved; a ringdown resonant cavity within the housing for holding a sample gas, saidcavity cell receiving said first order diffraction beam of said laserand comprising at least two high-reflectivity mirrors, wherein saidmirrors define an intracavity light path and one of said mirrors is amovable tuning mirror; a piezo transducer drive attached to the tuningmirror for modulating cavity length to maintain resonance between thelaser frequency and cavity modes; a photo-detector within said housing,for receiving said beam from the cavity and for generating a resonancesignal and a voltage decay (ring down) signal, thereby measuring aninteraction of said sample with said intracavity beam; and amicroprocessor for reducing the periodic noise in the voltage decaysignal by recording the cw-CRD voltage decay signals as data andsubjecting the data to an algorithm selected from either an averagingthe interquartile range of the data, or a cluster analysis.

In a preferred embodiment, the cavity has 4 mirrors in a bowtieconfiguration.

In another preferred embodiment, the trace species of the sample gas isselected from the group consisting of: HCHO, H2S, METHYL MERCAPTAN, CO2,CO, HCN, HCl, NH3, C2H2.

In another preferred embodiment, the trace species of the sample gas canbe large when using mid-IR lasers and may include trace species suchsarin, VX, mustard gas, arsine, phosgene, tear and pepper gases,explosives like TNT and other nitrogen-based explosives, andincapacitating agents such as B2.

Another preferred embodiment of the mid-IR CRDS apparatus furthercomprises a microprocessor for reducing the periodic noise in thevoltage decay signal by recording the cw-CRD voltage decay signals asdata and subjecting the data to an algorithm selected from either anaveraging the interquartile range of the data, or a cluster analysis.

In yet another preferred embodiment, the optical communication isoptical fiber based.

For a desktop apparatus, the foot print of this prototype would bebetween about 6″ to about 12″ wide, preferrably 8.5″ wide, by about 5″to about 8″ deep, preferably 6.5″ deep, by about 5″ to about 8″ tall,preferrably 4″ tall, as a bench-top device. In a preferred embodiment,the device weighs about 5 to about 12 pounds, and preferably weighingabout 6 pounds. Further, it can be easily configured to fit in a 2U boxfor standard 19″ rack.

Advances in the communications industry have produced inexpensive,reliable and robust diode lasers in the near infrared, thus providingone preferred embodiment of the inventive CRDS apparatus to be uniquelycompact and portable, and having low power consumption. These featuresare highly advantageous in a number of situations and allow thesensitivity of CRDS to be used in many novel approaches.

The inventive subject matter also includes a method for determining anexponential decay rate of a signal in a cavity ring down spectroscopicanalysis, said method comprising: providing a ring down resonant cavityfor holding a sample gas, wherein the cavity has at least one tunablemirror; illuminating the cavity with a tunable laser;

matching the cavity length to the laser frequency by moving the tunablemirror until resonance is detected; interrupting the laser beam;detecting one or more decay signals; and steps of: recording the decaysignals as data; and subjecting the data to an algorithm selected froman averaging of the interquartile range, wherein discarding the upperand lower quartiles before averaging the data values reduces theperiodic noise in cw-CRD spectra when using cavity modulation, or acluster analysis to reduce the periodic noise in cw-CRD spectra whenusing cavity modulation.

In alternative preferred embodiments, the tunable mirror comprises ahigh reflectivity mirror attached to a piezo transducer driver, or thetunable laser is a continuous wave laser, or the tunable laser is acontinuous wave near-infrared laser, or interrupting the laser beamcomprises switching off an acousto-optic modulator.

In a further preferred embodiment, the decay signals generated duringresonance number between about 1 kHz to about 20 kHz, and morepreferably between about 10 kHz to about 20 kHz.

Another preferred embodiment of the inventive method further comprisesthe step of: vi) maintaining resonance within the cavity by switchingthe AOM back on and monitoring the cavity for resonance, wherein decaysignals continue to be detected if resonance is detected within thecavity, and wherein cavity modulation by moving the tunable mirror isperformed if resonance is not detected within the cavity.

Another preferred instrument is uniquely compact and portable, and haslow power consumption. These features are highly advantageous in anumber of situations including monitoring of HCHO, H2S, METHYLMERCAPTAN, CO₂, CO, HCN, HCl, NH3, C2H2 (for both near-IR and mid-IR),and for monitoring sarin, VX, HCN, mustard gas, arsine, phosgene, tearand pepper gases, explosives like TNT, and incapacitating agents such asB2 (for mid-IR only).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic layout of one of the preferred embodiments of anexperimental setup.

FIG. 2. The top graph shows two cavity transmission modes seen whilemodulating the cavity length. The goal here is the left side, whichshows a single mode in an empty cavity. This can only result from astable optic design and good algorithms. The resulting ringdowns foreach transmission mode are shown in the bottom graph.

FIG. 3. A typical ringdown distribution acquired while taking a spectrumof CO₂. A normal distribution (ND) is calculated for each data set. Ashows a data set that is normal, while B has decays that are well beyondthe calculated normal values. The dashed line represents theinterquartile range (IQR) of a box and whiskers analysis. The differencein the normalized standard deviation from A to B is reduced from6.6×10⁻² to 1×10⁻⁴ with the box and whiskers analysis.

FIG. 4. Spectra obtained from a Wolfard-Parker burner at 9 mm HAB withand without the addition of pyridine to the fuel. the large peak to theleft is HCN, the next peak is C2H2 (acetylene).

FIG. 5. A series of CO₂ measurements obtained while varying the pressureand maintaining concentration. The 80, 40 and 20 torr peaks were fitwith a voigt lineshape while the 5 torr peak was fit with a gaussianlineshape. The fit determined concentration was 453 ppm with a standarddeviation of 7 ppm.

FIG. 6. A spectrum obtained with 50 torr laboratory air in the cavitywith residual NH₃.

Analog Figures

FIG. 7: The analog decay constant measurement. As the detector signalexceeds reference voltage 1, a voltage ramp is reset to zero. Thevoltage increases linearly during the time

t that the detector signal decays between the two reference voltages.The final voltage reached is proportional to the decay time of thesignal.

FIG. 8: A comparison of the sweep and hold procedure with continuoussweeping. The continuous sweep (top) produces resonance only at discretepoints in the sweep. The sweep and hold method (bottom) pauses the sweepat resonance and allows for the generation of a burst of ring-downs; thecavity mirrors are also held stationary during the measurement period.This allows sensitivity to greater than or equal to 1 KHz.

FIG. 9: The electronic block diagram: trigger 1 pauses the sweep andhold circuit, turns the AOM off, and initiates the analog ring-downtimer. Trigger 2 completes the ring-down measurement. The measurement isrecorded using an A/D converter and stored in memory. A microcontrollerincrements the laser wavelength after a predetermined number ofring-downs and sends data to an external computer.

FIG. 10: Cavity ring-down setup: (a) experimental layout and (b) mirrormount detail. The custom mirror mounts mate with standard CF fittings onthe gas sample cell.

FIG. 11: Sinusoidal oscillation of decay time with wavelength in anempty cavity, due to cavity mirrors behaving as etalons. Data points areshown as dots and a sinusoidal fit is shown as a solid line. This is anindication of the quality of the mirror, an aspect which is addressed bythe present invention. The variation is caused by reflection back fromthe back side of the mirror.

FIG. 12: (a) 1.7 ppm ammonia in air at 50 Torr; data points are shown asdots and fit is shown as a solid line (b) 28 ppm acetylene in air at 50Torr; data points are shown as dots and fit is shown as a solid line

FIG. 13: A comparison of decay times acquired by the analog measurementscheme with those acquired by a high speed analog to digital converter.Poor correlation between the two measurement schemes is a possibleindication of nonexponential decay. Note the two groupings of datapointswhich show analog (L) and digital (R).

FIG. 14: (a) Decay signal fit to an exponential and (b) thecorresponding residual. Note oscillations in the residual, indicative ofnonexponential decay.

FIG. 15 is a graph of a Cluster Analysis and shows a series of ringdownsthat are fit and a 2 dimensional Agglomerate clustering algorithm used,to plot decay constant (+10⁴) in units of sec−1 and fit quality(correlation coefficient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

CRDS

Cavity ring-down spectroscopy is a sensitive absorption measurementtechnique for trace gas detection. A measurement consists of observingthe rate of decay of light in a high finesse optical cavity, and thenrelating this rate of decay to the concentration of absorbing species.Two or more high reflectivity dielectric mirrors are arranged to form astable optical cavity. The finite transmission of the dielectriccoatings allows light to be introduced into the cavity through one ofthese mirrors. At some time t=0, the laser is switched off. The lightremaining in the cavity then decays with time due to a combination ofmirror losses and absorption losses. A detector measuring the intensityof light transmitted by the cavity is then used to measure the rate ofits decay. The absorbance at a given wavelength, α(λ), of gas inside thecavity is related to the decay time at that wavelength, τ_(λ), through

${{\alpha(\lambda)} = \frac{\tau_{c} - \tau_{\lambda}}{c\;\tau_{c}\tau_{\lambda}}},$

where τ_(c) is a measurement of the decay time of the cavity in theabsence of any absorption and c is the speed of light. This measurementmay be made by tuning the laser a few spectral line widths away from theabsorption feature of interest. It is assumed that the cavity lossesvary much more slowly with wavelength than does the moleculartransition. The concentration of the absorbing species may then bewritten in terms of the absorbance as

${x = \frac{\alpha(\lambda)}{{Sg}\;\rho}},$

where S is the line strength, g is a line shape factor, and ρ is thedensity of gas inside the cavity. A significant advantage of thistechnique over other types of absorption spectroscopy is that the decaytime depends solely on the optical properties of the cavity and istherefore independent of fluctuations in the laser intensity.

cw-CRDS

The cavity ring-down technique was originally developed using high powerpulsed lasers. Modifications to the original technique have made CRDSmeasurements possible using low power continuous wave lasers. Inaddition to reduced power requirements, the use of narrow line width cwsources also results in higher spectral resolution. When the laser linewidth is smaller than the free spectral range of the cavity, only asingle longitudinal mode of the cavity is excited. The ultimateresolution of the spectrometer is then determined by the line width ofthe cavity mode, which may be several orders of magnitude smaller thanthat of the laser. However, some means of matching the laser frequencyto a mode of the cavity is required; either the laser or the cavity mustbe adjusted to produce overlap between the laser output and a cavitymode. Meijer et al. reported using natural thermal and mechanicalinstabilities present in the cavity to produce random coincidencesbetween the laser frequency and a cavity mode. Rempe et al., Romanini etal., and He et al. each describe slightly differing methods ofmodulating the cavity length with a piezoelectric transducer in order toproduce periodic matches. Paldus et al. have developed a means ofactively locking the cavity length to match the laser frequency using afrequency tunable acousto-optic modulator (AOM).

AOM

Continuous wave CRDS also requires a fast switch, such as an AOM, tointerrupt the input beam once sufficient light has been stored in thecavity to observe its decay. The switch must act on a time scale that issmall compared to the decay time, which is on the order of 1 to 100 μswith the best available mirrors and cavity lengths on the order of ameter. Alternatively, the laser frequency may be quickly detuned to fallbetween two cavity modes. In the case of a diode laser, this can beachieved on the required time scale by a small change in the currentsupplied to the diode. The high finesse of the ring-down cavity ensuresthat virtually none of the detuned light is transmitted to the detector.

Data Capture

Once light has been successfully coupled into the cavity, the rate ofdecay of that light must be measured. A digital data acquisition schememay be used, in which signals from a detector are recorded with ahigh-speed digitizer and then fit to an exponential decay with astandard curve fitting routine. While accurate, this method iscomputationally intensive, and can become the limiting factor in thedata acquisition rate. Alternative analog methods of decay timemeasurement have been explored previously. Spence et al. use a simpleanalog computer in conjunction with a lock-in amplifier, a method bestsuited to a laser locked cavity where the ring-downs can be generated ata constant rate. Romanini and Lehmann make a two point measurement ofthe decay time using a boxcar averager. The boxcar is used to averagethe detector signal over two narrow windows separated by a fixed delaytime of Δt. A comparison of these two averages allows a determination ofthe decay time to be made. This method does not require a locked cavity.However, for best accuracy, the delay time Δt must be chosen in advanceto be on the order of the decay constant. If the decay time variessignificantly over the course of a spectral scan, as for example due tothe presence of a relatively strong absorption line, the accuracy of themeasurement will be reduced.

New Cavity Modulation and Ring Down Measurement Scheme

For the present invention, a cavity ring down spectrometer is providedthat incorporates a new type of cavity modulation and ring-downmeasurement scheme, and its advantages are discussed. Spectra of ammoniaand acetylene acquired using this arrangement are presented, and acomparison of ring-down measurements are made with measurements from adigital data acquisition card (Gage Applied Technologies CS1250).

Cavity Modulation—Mirror Displacement and Measurement Accuracy

One means of matching a mode of a ring-down cavity to a narrow linewidth source is by adjusting the cavity length. However, the resolutionmay be adversely affected when using a moving cavity mirror due to aDoppler shift of the light inside the cavity. In addition,nonexponential behavior of the decay signal may occur due tointerference effects.

Consider a cavity of length L. Suppose a longitudinal mode is excited bylight of frequency ν₀, and the light source is then shut off to allowthe cavity mode to decay. The order of the excited mode, m, is given by

${m = \frac{2{Lv}_{0}}{c}},$

where c is the speed of light. If one cavity mirror is moved at a speeds, the resonant frequency of the excited cavity mode, v_(c), changes by

${{v_{c}(t)} = \frac{mc}{2\left( {L + {st}} \right)}};\mspace{11mu}{\frac{mc}{2L}{\left( {1 - \frac{st}{L}} \right).}}$

The frequency of light inside the cavity will also change due to aDoppler shift caused by reflections from a moving mirror. This Dopplershift alters the frequency a factor of

$\frac{c - s}{c}$per reflection. Reflections occur at a rate

$\frac{c}{L},$, leading to a total frequency shift after a time t of

${v(t)} = {{v_{0}\left( \frac{c - s}{c} \right)}^{\frac{ct}{L}} = {\frac{mc}{2L}{\left( \frac{c - s}{c} \right)^{\frac{ct}{L}}.}}}$

Assuming that

$\begin{matrix}{{\frac{s}{c} = 1},} & \;\end{matrix}$this expression may be approximated, to first order, as

${{v(t)};\mspace{11mu}{\frac{mc}{2L}\left( {1 - \frac{st}{L}} \right)}},$

which is identical to the expression given by equation (4) for thechange in cavity mode frequency. Therefore, a cavity with a movingmirror shifts both the cavity mode frequency and the radiation frequencyby the same amount. As a result, resonance is maintained even if thefrequency shift due to cavity modulation exceeds the width of the cavitymode. Note that no assumption was made about the sign of the mirrorvelocity, so this derivation is applicable to both increasing anddecreasing cavity length.

In order to maintain the resolution of the spectrometer, the mirrorspeed should be such that the frequency shift due to length modulationover the decay time τ is small compared to the width of the cavity mode.A high finesse cavity with mirror reflectivity R has modes with a width,Δν, given by

${\Delta\; v} = {\frac{c\left( {1 - R} \right)}{2\pi\; L\sqrt{R}}.}$

From expressions (4) or (6), the mirror speed which produces a frequencyshift of Δν after a time τ can be found as

${\frac{{mcv}\;\tau}{2L^{2}} = \frac{c\left( {1 - R} \right)}{2\pi\; L\sqrt{R}}},$

or equivalently by

$v = {\frac{L\left( {1 - R} \right)}{\pi\; m\;\tau\sqrt{R}} = {\frac{c\left( {1 - R} \right)}{2\pi\;\tau\; v\sqrt{R}}.}}$

Considering only the losses due to finite reflectivity, the decay timemay be written as

${\tau = \frac{L}{c\left( {1 - R} \right)}},$

allowing equation (9) to be rewritten as

$v = {\frac{{c^{2}\left( {1 - R} \right)}^{2}}{2\pi\;{Lv}\sqrt{R}}.}$

This is the mirror speed that will produce a frequency shift after onedecay time equal to the cavity line width. As an example, for the cavityused in our experiments, L=0.5 m, R=0.9999, and λ=1.53 μm. The linewidth of the cavity is then found from equation (7) to be 9.6 kHz, andthe mirror speed calculated from equation (11) is 1.5 μm/s. Modulationover one free spectral range at this speed, requiring a back and forthmirror displacement of λ/2, would then occur at a rate of 1 Hz. This isthe modulation rate at which the Doppler shift is equal to the linewidth of the cavity. Modulation at a higher frequency or over a greaterdisplacement will produce a corresponding decrease in resolution.Although cavity line widths on the order of kilohertz are much narrowerthan typical thermally and collisionally broadened spectral features,the Doppler shift may become an important consideration forhigh-resolution spectroscopy.

Box & Whiskers—a Method to Reduce Periodic Noise

Light from a diode laser (distributed feedback or external cavity) isfocused onto an acousto optics modulator (AOM). The first orderdiffraction beam from the AOM is steered into an optical cavity using“mode matching” optics. The cavity length is modulated over one or morefree spectral range of the cavity using piezo actuator(s). Light exitingthe cavity is detected. Upon detection of “resonance: in the cavity(detector voltage over a threshold), the AOM is de-energized, thusstopping the flow of light into the cavity. The detector voltage decaysexponentially with a decay constant that is a function of the physicalparameters of the cavity (mirror losses and length) and spectralproperties of molecules that may be in the ring down cell. Several ringdown events are collected at each wavelength. Statistical analysis ofthe ensemble of ring down events using a variable width “box andwhiskers” sort improves precision. An entire spectrum may be collectedby scanning the wavelength. In this case, concentrations are determinedby fitting the spectrum to a simulate spectrum. To speed dataacquisition, a “sensor” mode has also been used. Here spectral baselinepoints are collected on either side of the analytical feature ofinterest and three spectral data pts are collected near the featurespeak. These latter three points are fit to a parabola to locate theabsolute peak maximum. The average of the baseline points determine theempty cavity decay constant. This data is correlated to the equivalentVoigt lien shape signal level to determine concentration. Finally, acorrection signal is applied to the laser current to recenter the peakwith respect to laser frequency. (due to drift).

Cluster Analysis

Another method involves Cluster Analysis. A series of ringdowns is fitand a 2 dimensional Agglomerate clustering algorithm is used, as shownin FIG. 15. Plotted below are decay constant (÷10⁴) in units of sec−1and fit quality (correlation coefficient. A number between 0 and 1 thatindicates how good the fit is.) For this data, there is a dominant modethat produces good fits and a fairly consistent decay constant.Clustering allows us to sort this data into common groupings. Thecircles indicates a possible groupings (base don the statistics). Thisis implemented in real time. It is a way to compensate for misalignmentsand multimode cavity excitation. It is even better than the box andwhiskers in that it does not assume a (normal) distribution around thepopulation median.

Near-Infrared and Mid-Infrared Lasers

Near-IR lasers range from about 1 to about 2.5 micrometers and are wellsuited for detection of smaller trace species such as HCHO, H2S, METHYLMERCAPTAN, CO2, CO, HCN, HCl, NH3, and C2H2.

In another preferred embodiment described herein, the mid-IR OPO laseruses a periodically poled lithium niobate (PPLN) crystal. This systemprovides better sensitivity, less interference, and can detect a broaderrange of molecules due to its higher power, e.g. 100-500 mWatts.

Experimental Setup

The schematic layout of this cw-CRDS experiment is shown in FIG. 1. Thelight source used is an external cavity diode laser (ECDL) manufacturedby New Focus (Velocity 6328). The ECDL has a continuous wavelength rangefrom 1510-1580 nm with a maximum power of 8 mW (typical 6 mW) and abandwidth of 5 MHz. Tuning the laser is accomplished by changing theangle between the tuning mirror and the grating (in the laser housing).A DC motor is used to course tune, while a piezo transducer (PZT)attached to the mirror is used to fine-tune the laser. A Burleighwavemeter (WA-1000) determines the wavelength of light emitted by thelaser throughout the scan.

A Gateway computer equipped with a Gage Compuscope 1250 PCI card wasused to control the ECDL PZT voltage and fine tune the laser. The cavitylength was modulated with the saw tooth function from a StanfordResearch Systems function generator (SRS345) driving a Thor Labs piezodriver (MDT691). The driver output was fed to three identical PZTssymmetrically surrounding the mirror to scan the ringdown cavity length.An InGaAs pin detector in a Los Gatos Research CRDS package was placedafter the second mirror. This detector was coupled with a currentfollower circuit to amplify the signal. When buildup occurredsufficiently to trigger the Gage card, the computer sent a three-voltsignal to a comparator circuit that de-energizes the ISOMETAcousto-Optic Modulator (thus shutting off the light into the cavity).The Gage card then captured the ringdown and stored it to a file. Foreach step in the spectrum 100-200 ringdowns were captured on the Gagecard and saved to the computer. Each ringdown was then fit to a singleexponential decay to determine the decay constant. After dataprocessing, the average decay was plotted versus wavelength to obtain aspectrum.

Mirrors with maximum reflectivity near 1.55 μm were obtained from LosGatos Research and were used for the ringdown cavity. Over the range ofwavelengths explored in this research, a slight wavelength dependence ofreflectivity (

) was observed as indicated by variation in empty cavity decay constants(typical values of 15 μs at 1540 nm and 12.5 μs at 1570 nm wereobtained). From these decay constants, reflectivity can be calculatedfrom

$\begin{matrix}{= {1 - \frac{L_{cavity}}{\tau \times c}}} & (2)\end{matrix}$

where L_(cavity) is the spacing between the mirrors, τ is the ringdowntime and c is the speed of light. The cavity length was 0.26 m resultingin calculated mirror reflectivities of 0.999940 and 0.999928 (1−

=60 ppm and 72 ppm) at 1540 nm and 1570 nm, respectively.

Although a three mirror triangular or four mirror rectangular cavity canbe utilized, in one preferred embodiment our instrument has a uniquefour mirror “bow-tie” cavity configuration. A bow-tie configurationprovides the following advantages. First, it allows for a more compactsensor. Second, it eliminates optical feedback to the laser source.Third, ring resonators are inherently more stable. Fourth, it providesfor a simplified alignment. Fifth, it allows for a longer opticalinteraction length. Sixth, it allows for uniformity of mirror optics.

Accordingly, the cavity consists of a high finesse resonator using fourmirrors (preferably piano-concave). As contemplated, the laser beamstrikes all four mirrors, making two passes through the cavity, i.e.,four passes for one round-trip. When using all plano-concave mirrors,all four mirrors can be identical and can, therefore, be fabricated in asingle coating run. The cost of a mirror coating run is high, so thissimplification in mirror optics significantly reduces the CRDS systemcost. As an alternative to four plano-concave mirrors, one can use twoflat and two plano-concave mirrors. Again, although the mirrorsubstrates would not all be identical, a single coating run could becarried out. In addition, if the input laser(s) do(es) not vary infrequency, only one of the mirrors of the cavity needs to be dithered toprovide a resonant cavity.

For a desktop apparatus, the foot print of this prototype would bebetween about 6″ to about 12″ wide, preferrably 8.5″ wide, by about 5″to about 8″ deep, preferably 6.5″ deep, by about 5″ to about 8″ tall,preferrably 4″ tall, as a bench-top device. In a preferred embodiment,the device weighs about 5 to about 12 pounds, and preferably weighingabout 6 pounds. Further, it can be easily configured to fit in a 2U boxfor standard 19″ rack.

Optimization of Experimental Variables and Data Analysis

Ideally, only one transverse mode of the laser beam should be excitedwith the cw-CRDS technique. This ensures that the laser beam is samplinga single location on the mirror surface. Sampling different locationsresults in complicated, multi-exponential ringdowns due to theinhomogeneous reflectivities of the surface of the mirror. A singletransverse mode structure allows for the buildup of light in the cavitywhen the laser is an integer multiple of the cavity's FSR. Modulatingthe cavity length over one FSR results in a build-up event at anywavelength throughout the scan. As noted above, cavity length modulationis accomplished using three identical piezo-electric transducers (PZT)so as to prevent compromising the alignment.

FIG. 2 shows the modulation of the cavity over a length corresponding tosomewhat greater than one FSR. In this figure the cavity is slightly outof alignment to show two sets of two transverse cavity modes separatedby one FSR. Further alignment of the cavity can select either of thetransverse modes (labeled A and B). Ringdowns for the single modeoperation were collected and are shown in FIG. 2B. As the data show, theringdown times for peaks A and B are distinctly different (10.6 and 9.4μs, respectively).

Multi-mode excitation can be minimized through careful cavity alignment.However, when ramping the cavity length over several FSRs, any mismatchin the PZTs may lead to excitation of undesired modes. This applies tousing three ganged drivers. Although the new annular piezos are free ofthis problem, statistical treatment of the data still helps.Fortunately, these extraneous points can be identified in analyzing thestatistics of the individual ringdown events. As a figure of merit forevaluating the distributions of ringdown curves, we track the standarddeviation normalized to the averaged ringdown time (σ/<τ>). FIG. 3Ashows the distribution of 94 ringdowns captured while taking a spectrumof CO₂ in air which is well described by a normal distribution. Ourfigure of merit, σ/<τ>, for this data was 3.6×10⁻³. FIG. 3B shows aringdown distribution that contains several ringdowns in which undesiredmode excitation occurred. For this set, σ/<τ> was substantially greater,6.8×10⁻². Romanini et al. also observed a “periodic noise” in theircw-CRD spectra when using cavity modulation. In their work, thisperiodic error is believed to be a result of a transmission mode excitednear the end of the a cavity scan.

A “box and whiskers” analysis was employed to eliminate the outliers andreduce periodic noise. In this procedure, the decay constants are sortedand the upper and lower quartile values are discarded leaving theinterquartile range (IQR), or middle fifty percent, to be averaged. Thenormalized standard deviation after a box and whiskers analysis(σ/<τ>IQR) results in a significant improvement in the agreement betweenthe two distributions: the averaged value of the IQR for FIG. 3A is1.6×10−3 while that of FIG. 3B which is 1.7×10−3. It is contemplatedherein that interquartile is not strictly limited to exactly the25^(th)-75^(th) percentiles, and can vary as low as about the 20^(th)percentile and about the 80^(th) percentile, as well as combinations andpermutations therebetween, e.g. 20-80, 20-75, 20-60, 25-75, 25-80,25-60, 30-75, 30-60, 30-80, as would be known to persons of ordinaryskill in this area.

Once a spectrum is obtained, it is fit with a simplex simulation routineto determine the mole fraction of the target species in the cell. Theabsorption coefficient (a) is calculated from

$\begin{matrix}{\alpha_{\lambda} = {\frac{\tau_{empty} - \tau_{\lambda}}{c \times \tau_{empty} \times \tau_{\lambda}} = {S \times g \times \rho \times x_{j}}}} & (3)\end{matrix}$

where τ is ringdown time, c is the speed of light, S is linestrength, gis lineshape, ρ is molecular density and x mole fraction of species j.The lineshape function in the fit can be Lorentzian, gaussian or Voigtdepending on the pressure inside the cavity. In order to fit the data,all the spectroscopic constants such as self-broadening, air-broadening,temperature dependence, pressure shifts, line strengths and linepositions are obtained from HITRAN 2000 for H₂O, CO and CO₂. The NH₃,C₂H₂ and HCN constants were obtained from the GIESA database or otherpublications and were optimized with the simplex fitting routine.

Results of cw-CRDS Spectra—Acetylene and Hydrogen Cyanide

For measurements of C₂H₂ and HCN, gas samples were extracted from amethane/air diffusion flame supported on a Wolfhard-Parker slot burner.This burner consist of an 8×41 mm fuel slot sandwiched between two 16×41mm air slots. A quartz microprobe that ran parallel to the slotseparators was used to sample the flame gases at 9 mm above the burner.At this flame height, two flame sheets are observed centered near 6.5 mmfrom the burner centerline. Both HCN and C₂H₂ concentrations were foundto peak in the hydrocarbon pyrolysis region inside of the flame sheets,approximately 4 mm from the centerline.

FIG. 4 shows a spectrum obtained in the flame near the peak in speciesconcentrations. In order to identify the peaks a simulation of acetyleneand hydrogen cyanide line positions was performed. The results werecompared to the acetylene standard reference material data sheets fromNIST to verify the accuracy of the simulation.

This confirmed the identity of a relatively weak acetylene absorptionfeature at 11539.735 nm. The GEISA database identified the peak at1539.695 as HCN. As a simple experiment to verify this peak as HCN, thefuel was bubbled though pyridine, adding approximately 1% dopant to thefuel flow. Pyridine pyrolyzes to HCN quantitatively, resulting in adramatic increase in peak intensity.

The values of C₂H₂ concentrations in the Wolfard-Parker burner have beenreported using a mass spectrometric technique, which allows for thecomparison with the calculated concentrations obtained with cw-CRDS. Theagreement of these two values is within a percent suggesting theaccuracy of our procedure.

Carbon Dioxide and Carbon Monoxide

To test the cw-CRD potential as a sensor for CO and CO₂, measurementswere made of the pure gas at the strongest peaks available around 1570nm that were free of interferences based on simulated spectra. Thedetection limits were then calculated by taking the standard deviationof the baseline to represent our noise level. A signal three times ashigh as the noise level resulted in detection limits of 2.0 ppm for COand 2.5 ppm for CO₂. A detection limit on this order for CO₂ allows forits detection in ambient air, resulting in an easy measurement thatrequires no gas dilution.

FIG. 5 shows the accuracy of the fitting routine on a CO₂ line at1572.33 nm (6360 cm⁻¹) in laboratory air. The line was fit with a Voigtlineshape for every pressure except 5 torr, for which a gaussian shapewas assumed. The average calculated concentration from the fits was 453ppm (σ=7.3 ppm).

Ammonia

NH₃ was injected into the empty cavity and then diluted with nitrogengas. The dilution was sufficient to take the concentration below ppmlevels in the cavity but due to the residual effects of NH₃ theresulting peak had a fit concentration well over 10 ppm. Without someother method of measuring NH₃ in the cavity we are unable to produce anaccurate calibration curve. In an attempt to determine detection limits,acetylene was substituted for NH₃ since it has comparable linestrengthsin this region. The C₂H₂ P4 line at 1527.44 was chosen because itscalculated linestrength of 2.37×10⁻²¹ cm⁻¹/(molecule·cm⁻²) closelyapproximates the largest NH₃ line which has a reported value of2.33×10⁻²¹ cm⁻¹/(molecule·cm⁻²). The calibration curve for this C₂H₂line with the cw-CRDS setup resulted in a linear slope ranging from 53ppm to 5 ppm. With the calibration of C₂H₂ it was possible to determinethe residual concentration of NH₃ detected when ambient air was sampled.FIG. 6 shows a CRDS spectrum of air at 50 torr where the NH₃ peaks areclearly visible. The peak maximum absorbance of NH₃ corresponds to aconcentration of 37 ppm.

Discussion of Results

When comparing the results from cw-CRDS to other absorption techniquesit is useful to relate detection limits in terms of the absorptioncoefficient. It has been reported that CRDS can produce detection limitsdown to 10⁻¹⁰ cm⁻¹ with suitable mirrors. With the current experimentalsetup and mirrors, absorption coefficients of 3×10⁻⁹ cm⁻¹ have beenachieved (3σ level). For other absorption techniques, where an increasein pathlength increases sensitivity, there must be a normalization ofthe absorption coefficient to compare it with CRD values. Direct,2f-wavelength modulation, and high frequency modulation absorptiontechniques have absorbance detection limits¹ on the order of 1×10⁻⁴,5×10⁻⁶ and 1×10⁻⁶ respectively. To achieve comparable absorptioncoefficient sensitivities as those attainable using CRDS (3×10⁻¹⁰), thepathlengths for these methods would need to be 3,333 meters for directabsorption, 167 meters for 2f modulation, and 33 meters for highfrequency modulation. ¹Table 1 shows the calculated detection limits (3σlevel) for our sensor and for the molecules discussed above [1].

Molecules cw-CRDS DL HCN 7.9 ppb CO 2.0 ppm CO₂ 2.5 ppm NH₃ 19.4 ppb H₂O1.8 ppm C₂H₂ 4.0 ppb

This cw-CRD system achieves sensitivities comparable to the bestfrequency modulation techniques requiring at least a 3-meter pathlengthcell.

Analog Electronics

In another preferred embodiment an analog circuit replaces some of thecontrol functions that used to be performed by external instrumentation.Analog allows for a smaller cavity and eliminates some of the hardwareand electronics required otherwise. As such, an analog system may tradesensitivity for cost or other factors, where sensitivity at certainlimits is not required. In this embodiment, voltage is generated on theboard that tunes the laser through an external laser diode currentsource and a voltage ramp is generated on the board to drive the cavitylength ramp (through an external piezo driver). The ramp is halted uponresonance n the hope that after a ring down event is recorded, thecavity will still be in resonance when the laser light is switched backon. In other words, bursts of events can be recorded. Although fulldigitization of the cavity decay curve may be used to determine thedecay constant, here, a two point sample and hold circuit is used todetermine the decay constant. A circuit board that implements thesefunctions has been designed and breadboarded, including the use of aBasic Stamp microcontroller. Having the full ring down curve (and theirexponential fits) allows not only a statistical advantage (severalhundred data points determine each decay constant), but also allow thedetection of “fault modes”: multiexpoential decays, ineffectiveshuttering, etc. Although this can be implemented using a sophisticateddigitizing card mounted in a PC, it can also utilize a Megasample persecond embedded Analog to digital microcontrollers to do the job.

An analog detection scheme is used to measure the decay constant of thering-down signals, requiring only a single voltage to be digitized andstored per ring-down. Curve fitting routines common to traditionaldigital methods are eliminated, allowing for measurement speeds limitedonly by the finite build up and decay time of light within the cavity.Near infrared spectra of ammonia and acetylene were obtained with thisspectrometer with an extinction coefficient detection limit of 2×10⁻⁸cm⁻¹, equivalent to 180 ppbV for ammonia and 500 ppbV for acetylene forthe specific lines investigated, at a pressure of 50 Torr. A comparisonis made between ring-down data taken with this scheme and with a digitaldata acquisition card. From this comparison, the dominant source ofnoise appears to be non-exponential decay due to beating betweenmultiple transverse modes.

Experimental Procedure

In this preferred embodiment, both the ring-down measurement and thecavity modulation strategy have been implemented with a custom builtelectronic circuit. A microcontroller is used for data processing,control of the laser frequency, and to send data to an externalcomputer. A schematic of the electronics is shown in FIG. 9.

Spectra were acquired out using an external cavity diode laser (ECDL)operating at a wavelength of 1.53 μm (New Focus model 6328). The ECDLemploys a Littman type laser cavity in which frequency changes are madeby moving a tuning mirror. A piezoelectric transducer mounted on theback of the tuning mirror is used for fine frequency adjustments, andmodulation of this transducer is limited to 2 kHz. The observed decaytimes are on the order of 15 μs. Thus, frequency detuning as ashuttering mechanism was not possible due to the slow response time ofthe external cavity. Therefore, an AOM is used to interrupt the beamwhen resonance is reached to allow for the decay of light in the cavity.

AOM

The AOM used in the experiments (Isomet Corporation model 1205-C1) has aswitching time of tens of nanoseconds, providing a sufficiently sharpcutoff of the beam for ring-downs lasting several microseconds. When theAOM is energized, a fraction of the incident light is diffracted intothe first order beam through an angle of 34 mrad with respect to thezeroth order beam. The first order beam is used to excite the cavity,and is turned off by de-energizing the AOM.

Layout

The experimental layout is shown in FIG. 10. A linear ring-down cavityis formed between a pair of 2 cm diameter mirrors of 99.99%reflectivity, spaced 0.5 m apart. Each mirror has a concave surface,with a 6 m radius of curvature. This forms a stable optical resonatorwith a free spectral range of 300 MHz. The mirrors are sealed withO-rings against 34 mm CF flanges, which are in turn welded to either endof a 1.9 cm (0.75″) diameter stainless steel tube. The tube forms a gassample cell, and is provided with three ports for gas inlet, exhaust,and a pressure tap. The mirror mounts are custom made assembliesdesigned to mate with the CF flanges. A schematic of the experimentalarrangement and a detail of the mirror mounts are shown in FIG. 4Analog.

Longitudinal modes of the cavity are matched to the laser frequencyusing an annular piezo-electric transducer (Physik Instrumente modelP-0016.00H) to modulate the length of the cavity by a few free spectralranges. This guarantees that an arbitrary laser frequency will match acavity mode at some point during the modulation cycle. The transducer ismounted directly behind one cavity mirror, with the input beam passingthrough the hole in its center. The O-ring on the other side of themirror provides the necessary compression to hold the mirror against thePZT, seals the mirror against the cavity, and allows for the smallamount of movement required to modulate the cavity length.

“Sweep & Hold”—Cavity Modulation

In order to increase the measurement speed, coupling of light into thecavity is enhanced by a ‘sweep and hold’ cavity modulation technique.Matching of the cavity length to the laser frequency is accomplished bymoving one of the cavity mirrors with a piezoelectric transducer.Resonance is detected by observing the build up of light in the cavity.When the intensity reaches a preset threshold, the AOM is switched offto interrupt light into the cavity and the PZT scan is paused. After thedecay of light is recorded, the AOM is switched on. If the laserfrequency and cavity mode are still resonant, light again builds up inthe cavity. This process is repeated as long as resonance is maintained.If light fails to build up in the cavity after a few hundredmicroseconds, scanning of the mirror is resumed. This process is shownpictorially in FIG. 8. Rather than a single ring down each time thecavity is brought into resonance, a burst of ring downs is generated.This burst lasts until resonance is lost due to drift in the laser ormechanical and thermal instabilities in the cavity. During the resonanceperiod, ring-downs may be generated at a rate limited only by the finitebuild up and decay time of the cavity itself, on the order of tens ofkilohertz. In preferred embodiments, about 1 to about 10 kHz, and morepreferably about 10 to about 20 kHz.

In experiments, each burst of ring-downs typically lasts for no morethan 5-10 ms, although on occasion they may last for several hundredmilliseconds. It should be noted that although we were able to generatering-downs at ten kilohertz, the speed of the random access memory usedby our microcontroller limited the recording of decay times to a rate ofonly 1 kHz. The sweep and hold method has the added advantage of holdingthe cavity length constant during each ring down, as opposed to thecontinuous sweep method. This eliminates the problems caused by a movingcavity mirror, and thus an arbitrary modulation frequency may be usedwithout compromising the resolution or accuracy of the spectrometer.

Alignment of the cavity is performed manually with three adjustmentscrews on each mirror mount. Mode matching optics and a spatial filterare used to match the input beam to the TEM₀₀ mode of the cavity,although there was still some difficulty in suppressing multimodeexcitation. It should be noted that we could not directly determinewhether the TEM₀₀ mode is in fact the dominant mode in the cavity. Thereare usually several transverse modes that are excited over one freespectral range, even with careful alignment. A threshold level isadjusted to ensure that only ring-downs from the dominant mode aremeasured.

Analog Time Constant Measurement

Light transmitted by the cavity is detected with an InGaAs photodiode(Thorlabs model FGA10). The signal from this photodiode is amplifiedwith a transimpedance amplifier having a gain of 1 V/μA. Ring-downmeasurements begin when the detector signal exceeds a preset thresholdlevel. This causes a trigger signal to de-energize the AOM, interruptingthe input beam. The decay of light remaining in the cavity is thenmeasured with an analog circuit using a two-point measurement based onthe voltage level of the signal. The measurement is made with a linearvoltage ramp produced as the signal falls between two referencevoltages, as illustrated in FIG. 7. The final voltage reached by theramp is proportional to the decay time. The second reference voltage isfixed at one third of the first, so the measurement takes place overln(3)=1.1 time constants. Note that the measurement always takes placein the same fraction of a time constant. This allows optimal accuracy tobe maintained over a broad range of decay times. The final voltagereached by the ramp is directly proportional to decay time and thusrequires only a single value to be digitized and stored per ring down.This increases the maximum rate at which data can be taken and greatlyreduces data processing requirements when compared to digital methods.

Results

Spectra of ammonia and acetylene at several ppmv concentration in airwere obtained to test the measurement and control procedure. The lowconcentrations were achieved by successive dilutions with room air.Laser frequency calibration was done with a wavemeter (BurleighInstruments model WA-1000).

It was found that the decay time of the cavity exhibits sinusoidaloscillations with wavelength. The amplitude of oscillation is 1.8 μs,and the period is 0.118 nm. This period corresponds to that of an etalonwith the same thickness as the mirrors used in the cavity, and is mostlikely due to the finite reflectivity of the back surfaces of themirrors. This causes the mirrors to behave as etalons as the laserwavelength is scanned through their free spectral range. Theoscillations are illustrated in FIG. 11. A baseline scan was taken withroom air, and the result is shown in the figure with a sinusoidal fit.Because they are attributed to mirror behavior and do not representabsorption, these oscillations have been fit to a sinusoidal baselinedecay time in the analysis of spectral data.

Examples of ammonia and acetylene spectra are shown in FIG. 12, takenusing the experimental setup shown in FIG. 7. These spectra were takenat room temperature and 50 Torr cavity pressure. This pressure is chosenas a compromise between minimizing pressure broadening and maximizingpeak absorbance. One hundred ring-downs were taken at each spectralpoint and averaged. The best detection limit achieved with this systemis 2×10⁻⁸ cm⁻¹ with 100 ring-down averaging, based on a signal to noiseratio of 3.

Also shown in FIG. 12 are spectral fits to the data. For ammonia, thesespectral fits were generated using a Voight profile with line strengthdata taken from Webber et al. The ammonia lines are among thoserecommended by Webber et al for air quality monitoring within the 2ν₃overtone and (ν₁+ν₃) combination bands: the ^(P)P₃(5)_(a) line at6528.764 cm⁻¹, and an unassigned line at 6528.894 cm⁻¹. The linestrengths are 2.53×10⁻²¹ cm³/(molecule×cm²) and 1.34×10⁻²¹cm³/(molecule×cm²), respectively. The line assignment is taken fromLundsberg-Nielsen et al. Acetylene has several vibrational bands in thenear infrared spectral region that are either Σ

Σ or Π

Σ. The strongest lines are combinations of strong C—H stretching modes.A NIST frequency standard using acetylene capitalizes on the regularprogression of the 1010⁰0⁰

0000⁰0⁰ (ν₁+ν₃) combination band. In addition, several hot bands existin which the lower states have populations in low frequency bendingmodes leading to several weaker transitions evident in thehigh-resolution spectrum between the larger features of the ground statetransitions. Line strengths for these features can be determined byfitting the high-resolution spectrum of the NIST SRM to a Voight lineshape using the conditions of this gas sample. For the features shown inFIG. 12, these lines are 6494.51 cm⁻¹ and 6494.62 cm⁻¹, with linestrengths of 4.70×10⁻²² cm³/(molecule×cm²) and 1.83×10⁻²²cm³/(molecule×cm²), respectively. Due to variability in the ECDL tuningbetween successive scans, there is up to 0.1 cm⁻¹ offset error in thefrequency axis. This error has been removed using the published linepositions for the selected ammonia and acetylene lines shown in FIG. 12.

Internal Comparison—Digital vs. Analog

In order to evaluate the performance of the two-point measurementscheme, ring-down measurements were compared with measurements made by adigital data acquisition system. The digital measurements were obtainedusing a high-speed digitizer (Gage Applied Technologies, Inc. modelCS1250), and then fitting the data to a single exponential decay. Theanalog circuitry and the digitizer measured each ring-downsimultaneously in order to provide a direct comparison of the twomethods. The spread in the decay times acquired using each method isplotted in FIG. 13 for 200 ring-downs. The standard deviation in thedata between the two methods is about the same, but the noise is notwell correlated between them. If it were, all the data should fall on astraight line with a slope of unity. The fact that it does not isindicative of a deviation from the assumed exponential behavior of thering down signal.

It was discovered upon examination of the digitally recorded signalsthat most of the ring-down events are not single exponential decays, butrather contain an oscillating component superimposed on an exponentialdecay. The amplitude of this oscillation varies between differentring-downs, and may account for the discrepancy in ring down valuesfound from the two measurement methods. The residual between therecorded signal and an exponential fit for a representative ring-down isshown in FIG. 14. The period of the fluctuations is about half of onedecay constant, corresponding to a time of 6 μs. Similar oscillationshave also been noted by Romanini et al. and by Paldus et al. Romaniniattributed this to beating between different transverse modes atslightly different frequencies, and we believe the same to be the casein our experiments.

Suppression of spurious transverse modes by proper mode matching isimportant in eliminating this source of noise. One limitation for modematching with the current experimental setup is the use of an AOM toswitch the beam on and off. The AOM has a wavelength dependentdeflection angle, which for the first order diffracted beam is given bythe expression

${\theta = \frac{\lambda\; f}{v_{s}}},$

where λ is the laser wavelength, f is the acoustic frequency, and v_(s)is the sound speed in the modulator crystal. The propagation angle ofthe input beam will change linearly with wavelength during the course ofa spectral scan due to this effect, introducing a slight alignment errorof the beam at the input mirror of the cavity. As pointed out by Wheeleret al., transverse mode beating effects can also be minimized byfocusing the entire cross section of the output beam onto the detector.

Thus, a new method of cavity ring-down time constant measurement andmode matching technique has been provided. This allows for the lowerdata processing and power requirements with increased acquisition speedsuseful for more compact and energy efficient ring-down spectrometerssuitable for portable applications. In a portable device, the analogcircuit is incorporated into the instrument design. For example, astand-alone function generator would not be used to create ramps, butrather these are built in.

Detection limits for ammonia and acetylene for the spectral lines usedin this investigation were 180 ppbV and 500 ppbV, respectively. In thecase of acetylene, the measurement was based on relatively weaktransitions in the (ν₁+ν₃) combination band. Spectroscopy based on thestrongest line in the (ν₁+ν₃) band would allow for a reduction inmeasurable concentration to 17 ppbV at the same extinction coefficient.

A potential advantage of Approach One is that having the full ring downcurve (and their exponential fits) allows not only a statisticaladvantage (several hundred data points determine each decay constant),but also allow the detection of “fault modes”: multiexpoential decays,ineffective shuttering, etc.

EXAMPLES Example 1

A CRDS apparatus using near-IR as described is installed in an aircraftto monitor trace species relevant to fire detection. It would beexpected that there would be a significant improvement in reducing thenumber of false alarms which cause mandatory grounding and inspection ofaircraft and thus generate a significant savings in aircraft maintenancecosts.

Example 2

A CRDS apparatus using near-IR as described is installed in a commercialoffice building, e.g. the HVAC system to monitor trace species relevantto fire detection. It would be expected that there would be asignificant improvement in reducing the number of false alarms and inimproving the sensitivity of such fire detection devices and thusgenerating a significant savings in building maintenance and safetycosts.

Example 3

A CRDS apparatus using mid-IR as described is installed in an aircraftto monitor trace species relevant to air quality and safety, e.g.chemical weapons. It would be expected that there would be a significantimprovement in improving the detection of large molecule toxins as wellas in reducing the number of false alarms which cause mandatorygrounding and inspection of aircraft and thus generate a significantsavings in aircraft maintenance costs.

Example 4

A CRDS apparatus using mid-IR as described is installed in a commercialoffice building, e.g. the HVAC system to monitor trace species relevantto air quality and public safety, e.g. chemical weapons. It would beexpected that there would be a significant improvement in improving thedetection of large molecule toxins as well as reducing the number offalse alarms of such air sample detection devices and thus generating asignificant savings in building maintenance and safety costs.

Example 5

A CRDS apparatus using either mid-IR or near-IR as described isinstalled in a public transportation system or vehicle to monitor tracespecies relevant to fire detection. It would be expected that therewould be a significant improvement in reducing the number of falsealarms and in improving the sensitivity of such fire detection devicesand thus generating a significant savings in transportation maintenanceand safety costs.

Example 6

A CRDS apparatus using either mid-IR or near-IR as described herein isinstalled in a public transportation system or vehicle to monitor tracespecies relevant to air quality and safety, e.g. chemical weapons. Itwould be expected that there would be a significant improvement inimproving the detection of large molecule toxins as well as in reducingthe number of false alarms which cause mandatory shut-down andinspection of transportation systems and thus generate a significantsavings in maintenance costs.

It will be clear to a person of ordinary skill in the art that the aboveembodiments may be altered or that insubstantial changes may be madewithout departing from the scope of the invention. Accordingly, thescope of the invention is determined by the scope of the followingclaims and their equitable Equivalents.

1. A compact cavity ring down spectroscopy apparatus for detection andmeasurement of trace species in a sample gas, which comprises: a housingfor said apparatus; a tunable solid-state continuous-wave mid-infraredPPLN OPO laser within said housing; an acousto-optic modulator in fiberoptical communication with said laser for steering a first orderdiffraction beam of said laser and for interrupting said beam whenresonance is achieved; a ring down resonant cavity within the housingfor holding a sample gas, said cavity cell receiving said first orderdiffraction beam of said laser and comprising at least twohigh-reflectivity mirrors, wherein said mirrors define an intracavitylight path and one of said mirrors is a movable tuning mirror, whereinthe cavity has 4 mirrors in a bowtie configuration; a piezo transducerdrive attached to the tuning mirror for modulating cavity length tomaintain resonance between the laser frequency and cavity modes; and aphoto-detector within said housing, for receiving said beam from thecavity and for generating a resonance signal and a voltage decay (ringdown) signal, thereby measuring an interaction of said sample with saidintracavity beam; and, a microprocessor for reducing the periodic noisein the voltage decay signal by recording the cw-CRD voltage decaysignals as data and subjecting the data to a cluster analysis oraveraging the interquartile range of the data; wherein the trace speciesof the sample gas is selected from the group consisting of: HCHO, H2S,methyl mercaptan, CO2, CO, HCN, HCl, NH3, C2H2, sarin, VX, mustard gas,arsine, phosgene, tear gas, pepper gas, nitro-based explosive, and B2;and, wherein the apparatus is configured as a desktop apparatus whereinthe foot print is between about 6″ to about 12″ wide, by about 5″ toabout 8″ deep, by about 5″ to about 8″ tall.
 2. The apparatus of claim1, configured as a desktop apparatus wherein the foot print is betweenabout 8.5″ wide, by about 6.5″ deep, by about 4″ tall.
 3. The apparatusof claim 1, configured to weigh about 5 pounds to about 12 pounds. 4.The apparatus of claim 1, configured to weigh about 6 pounds.
 5. Theapparatus of claim 1, configured to fit in a 2U box for standard 19″rack.
 6. A method for determining an exponential decay rate of a signalin a cavity ring down spectroscopic analysis, said method comprising:providing the apparatus of claim 1 a ring down resonant cavity forholding a sample gas, wherein the cavity has at least one tunablemirror; illuminating the cavity with said tunable solid-statecontinuous-wave mid-infrared PPLN OPO laser; matching the cavity lengthto the laser frequency by moving the tunable mirror until resonance isdetected; interrupting the laser beam; and detecting one or more decaysignals.
 7. The method of claim 6, wherein the tunable mirror comprisesa high reflectivity mirror attached to a piezo transducer driver.
 8. Themethod of claim 6, wherein interrupting the laser beam comprisesswitching off an acousto-optic modulator.
 9. The method of claim 6,wherein the decay signals generated during resonance number betweenabout 1 kHz to about 20 kHz.
 10. The method of claim 6, wherein thedecay signals generated during resonance number between about 10 kHz toabout 20 kHz.
 11. The method of claim 6, further comprising the step of:maintaining resonance within the cavity by switching the AOM back on andmonitoring the cavity for resonance, wherein decay signals continue tobe detected if resonance is detected within the cavity, and whereincavity modulation by moving the tunable mirror is performed if resonanceis not detected within the cavity.
 12. The method of claim 6, furthercomprising the steps of: recording the decay signals as data; and andsubjecting the data to a cluster analysis to reduce the periodic noisein cw-CRD spectra when using cavity modulation.
 13. The method of claim6, further comprising the steps of: recording the decay signals as data;and averaging the interquartile range, wherein discarding the upper andlower quartiles before averaging the data values reduces the periodicnoise in cw-CRD spectra when using cavity modulation.